Nowadays, in consideration of international environmental policies, reduction of the power consumption of electronic devices is desired. For example, a high-frequency amplification device for amplifying transmission signals is present in the final stage of a transmission section of a base station for a mobile telephone system, and reduction of the power consumption of the high-frequency amplification device is desired.
In general, a high-frequency amplification device has low power efficiency. An envelope-tracking amplification device is an amplification device for improving power efficiency. The envelope-tracking amplification device supplies an amplifier with power supply voltage corresponding to the amplitude of an envelope of a signal to be amplified by the amplifier, thereby reducing power loss.
As an example, when the amplitude of an envelope of a signal to be amplified is less than or equal to a certain threshold value the envelope-tracking amplification device supplies the amplifier with only fixed-voltage power generated by a high-efficiency power supply. When the amplitude of an envelope of a signal to be amplified exceeds the certain threshold value, the envelope-tracking amplification device supplies the amplifier with, in addition to the fixed-voltage power, variable-voltage power that responds to (tracks) the envelope of the signal to be amplified and uses a low-efficiency power supply. Thus, by changing the power supply based on the value of an envelope of a signal to be amplified, the envelope-tracking amplification device improves the power efficiency of the amplification.
In an amplifier of the envelope-tracking amplification device, for example, a laterally diffused metal oxide semiconductor (LD-MOS) composed of silicon, a field-effect transistor (FET) using gallium arsenide (GaAs) or gallium nitride (GaN) as a material thereof, a high electron mobility transistor (HEMT), or the like is used. In particular, a GaN-FET is superior to other devices in terms of saturation performance, high-frequency gain, and power efficiency, and therefore is increasingly being used in amplifiers in recent years.
However, a GaN-FET has a characteristic called “current collapse” where drain current decreases. For example, when a GaN-FET's drain voltage is high and gate voltage is low, electrons are trapped around the GaN-FET's gate. When electrons are trapped, the electron density in the GaN-FET decreases and the channel resistance increases. Therefore, the drain current of the GaN-FET decreases. For an amplifier using a GaN-FET, the current collapse decreases the degree of amplification and increases distortion of the signal.
In order to release trapped electrons, energy for causing the electrons to pass through a barrier may be supplied to the electrons. For example, the GaN-FET and a light-emitting device are optically coupled with each other and energy is supplied to the trapped electrons by radiating light onto the GaN-FET from the coupled light-emitting device. Thus, an increase in the on-resistance (that is, a decrease in current) due to the current collapse may be addressed.
A group-III nitride semiconductor FET and a group-III nitride semiconductor field-effect semiconductor device, in which a light source for radiating light onto the FET and a driving circuit for the light source are reduced in size, have been proposed (for example, refer to Japanese Laid-open Patent Publication No. 2008-47767).
In addition, a semiconductor device capable of reducing the effects of current collapse in an FET with high effectiveness has also been proposed (for example, refer to Japanese Laid-open Patent Publication No. 2008-198731).
However, regardless of the energy for addressing the current collapse, one problem is that power is wastefully consumed when light is radiated in accordance with the on or off state of an amplifier.
For example, when light with intensity higher than an intensity desired to release trapped electrons is radiated while the amplifier is turned on, power is wastefully consumed.